A DAC is a worthwhile upgrade. I am reviewing currently a $5K DAC that reveals the improvements over the years (about 3 years) in DAC performance. The change to performance is as strong as any preamp, amp or set of cables. Imo, if you are very serious about advancing your system and listening, it is defensible to upgrade the DAC every 3-5 years. If I did not review I would question the reports of how much DAC improvement there has been. But, the results are overwhelming, undeniable - and that's based on building many rigs for comparison, not just one. In the current comparison to the 3 year old DACs the difference is striking, imo well worth upgrading.
Speakers are limited by the upstream electronics, so you can get different, and even "better", but don't expect a speaker change to magically make the components leapfrog their absolute limits. The components will not magically morph into upper end gear. Do speakers if you have a lower end set and wish to elevate substantially within the limitations of the components, or if you want a sea change to a different genre of speaker.
Speakers are limited by the upstream electronics, so you can get different, and even "better", but don't expect a speaker change to magically make the components leapfrog their absolute limits. The components will not magically morph into upper end gear. Do speakers if you have a lower end set and wish to elevate substantially within the limitations of the components, or if you want a sea change to a different genre of speaker.